U.S. patent number 5,598,506 [Application Number 08/258,579] was granted by the patent office on 1997-01-28 for apparatus and a method for concealing transmission errors in a speech decoder.
This patent grant is currently assigned to Telefonaktiebolaget LM Ericsson. Invention is credited to Rolf A. Bergstrom, Karl T. Wigren.
United States Patent |
5,598,506 |
Wigren , et al. |
January 28, 1997 |
Apparatus and a method for concealing transmission errors in a
speech decoder
Abstract
A receiver in a frame based radio communication system includes
a speech decoder of the source-filter type which is controlled by
internal state variable updated on a frame by frame basis for
modifying received filter parameters representing background sounds
transmitted over a communication channel. The receiver detects
frames containing transmission errors, decides whether a frame in
which transmission errors have been detected is acceptable, and
conceals the detected transmission errors by restricting updating
of at least one of the internal state variables if the detected
frame is declared non-acceptable.
Inventors: |
Wigren; Karl T. (Uppsala,
SE), Bergstrom; Rolf A. (Molndal, SE) |
Assignee: |
Telefonaktiebolaget LM Ericsson
(Stockholm, SE)
|
Family
ID: |
20390257 |
Appl.
No.: |
08/258,579 |
Filed: |
June 10, 1994 |
Foreign Application Priority Data
|
|
|
|
|
Jun 11, 1993 [SE] |
|
|
9302025 |
|
Current U.S.
Class: |
704/233; 704/228;
704/E19.003 |
Current CPC
Class: |
G10L
19/005 (20130101); H04L 1/0061 (20130101); G10L
25/78 (20130101) |
Current International
Class: |
G10L
19/00 (20060101); G10L 11/02 (20060101); G10L
11/00 (20060101); H04L 1/00 (20060101); G10L
005/06 (); G10L 009/00 () |
Field of
Search: |
;395/2.35,2.36,2.42,2.37
;371/36,48,54 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
L R. Rabiner, "Applications of Voice Processing to
Telecommunications", Proc. IEEE, vol. 82, No. 2, pp. 199-228. .
International Publication No. WO89/08910. .
J. G. Proakis, "Digital Communication", 2nd Ed, McGraw-Hill, 1989
pp. 220-234, 362-365, and 593-624. .
G. C. Clark et al., "Error Correction Coding for Digital
Communication," Plenum Press 1981, pp. 1-17, 49-53, 73-75 and
227-235. .
T. B. Minde et al., "Techniques for Low Bit Rate Speech Coding
Using Long Analysis Frames", ICASSP, Minneapolis, USA, 1993. .
B. S. Atal et al., "Advances in Speech Coding", Kluwer Academic
Publishers, 1991 pp. 69-79. .
"Voice Activity Detection", Recommendation GSM 06.32, ETSI/GSM,
1991. .
IEE colloquium od `Terrestrial DAB--Where is it going`, p. 3/1-8,
Plange et al., "Combined channel coding and concealment". Feb.
1993. .
IEEE transactions on communications, vol. 37 Issue 3, Bryden et
al., "Error correction/masking for digital voice transmission over
the land mobile satellite system", pp. 309-341. Mar. 1989. .
ICASSP '89: Acoustics, Speech and Signal processing conference,
Kobatake et al., "Speech/Nonspeech discrimination for speech
recognition system under real life noise environments", pp.
365-368. Jan. 1989. .
IEE Colloq. (1988) No. 139: Digitized Speech Communication via
Mobile Radio, Freeman et al., "A voice activity detector for the
pan-European digital cellular mobile telephone service", pp.
6/1--6/5. Jan. 1988..
|
Primary Examiner: MacDonald; Allen R.
Assistant Examiner: Dorvil; Richemond
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis,
L.L.P.
Claims
We claim:
1. An apparatus in a receiver in a frame based radio communication
system, for concealing transmission errors in a speech decoder
caused by a communication channel, which speech decoder is of the
source-filter type and is controlled by means including internal
state variables updated on a frame by frame basis for modifying
received filter parameters representing background sounds
transmitted over said communication channel, said apparatus
comprising:
(a) means for detecting frames containing transmission errors;
(b) means for deciding whether a frame in which transmission errors
have been detected is acceptable;
(c) means for concealing said detected transmission errors by
restricting updating of at least one of said internal state
variables of said speech decoder if said detected frame is declared
non-acceptable by said deciding means.
2. The apparatus of claim 1, wherein said filter parameter
modifying means includes a voice activity detector with at least a
threshold for speech/background sound decisions, said concealing
means restricting updating of said threshold if said detected frame
is declared non-acceptable by said deciding means.
3. The apparatus of claim 2, wherein said filter parameter
modifying means further includes a stationarity detector connected
to an output of said voice activity detector for discriminating
between stationary and non-stationary background sounds, said
stationarity detector including at least one buffer containing
estimates of statistical moments of recent frames dominated by
background sounds for making stationarity decisions, said
concealing means restricting updating of said buffer if said
detected frame is declared non-acceptable by said deciding
means.
4. The apparatus of claim 2, wherein said filter parameter
modifying means further includes a stationarity detector connected
to an output of said voice activity detector for discriminating
between stationary and non-stationary background sounds, said
concealing means inhibiting updating of the
stationarity/non-stationarity decision obtained from the previous
frame if said detected frame is declared non-acceptable by said
deciding means.
5. The apparatus of claim 2, wherein said filter parameter
modifying means includes means for low pass filtering of the filter
parameters, said concealing means restricting updating of filter
coefficients of the low pass filtering process if said detected
frame is declared non-acceptable by said deciding means.
6. The apparatus of claim 5, comprising a post filter for modifying
the tilt of the spectrum of the decoded signal, said concealing
means restricting updating of tilt information if said detected
frame is declared non-acceptable by said deciding means.
7. The apparatus of claim 2, wherein said filter parameter
modifying means includes means for bandwidth expansion of the
filter represented by the filter parameters, said concealing means
restricting updating of filter coefficients if said detected frame
is declared non-acceptable by said deciding means.
8. The apparatus of claim 7, comprising a post filter for modifying
the tilt of the spectrum of the decoded signal, said concealing
means restricting updating of tilt information if said detected
frame is declared non-acceptable by said deciding means.
9. The apparatus of claim 1, wherein said filter parameter
modifying means includes a voice activity detector for performing
speech/background sound decisions on a frame by frame basis, said
concealing means inhibiting updating of the speech/background sound
decision obtained from the previous frame if said detected frame is
declared non-acceptable by said deciding means.
10. The apparatus of claim 9 wherein said filter parameter
modifying means further includes a stationarity detector connected
to an output of said voice activity detector for discriminating
between stationary and non-stationary background sounds, said
stationarity detector including at least one buffer containing
estimates of statistical moments of recent flames dominated by
background sounds for making stationarity decisions, said
concealing means restricting updating of said buffer if said
detected frame is declared non-acceptable by said deciding
means.
11. The apparatus of claim 9, wherein said filter parameter
modifying means further includes a stationarity detector connected
to an output of said voice activity detector for discriminating
between stationary and non-stationary background sounds, said
concealing means inhibiting updating of the
stationarity/non-stationarity decision obtained from the previous
frame if said detected frame is declared non-acceptable by said
deciding means.
12. The apparatus of claim 9, wherein said filter parameter
modifying means includes means for low pass filtering of the filter
parameters, said concealing means restricting updating of filter
coefficients of the low pass filtering process if said detected
flame is declared non-acceptable by said deciding means.
13. The apparatus of claim 9, wherein said filter parameter
modifying means includes means for bandwidth expansion of the
filter represented by the filter parameters, said concealing means
restricting updating of filter coefficients if said detected flame
is declared non-acceptable by said deciding means.
14. A method in a receiver in a frame based radio communication
system, for concealing transmission errors in a speech decoder
caused by a communication channel, which speech decoder is of the
source-filter type and comprises means including internal state
variables updated on a frame by frame basis for modifying received
filter parameters representing background sounds transmitted over
said communication channel, said method comprising:
(a) detecting frames containing transmission errors;
(b) deciding whether a frame in which transmission errors have been
detected is acceptable;
(c) concealing said detected transmission errors by restricting
updating of at least one of said internal state variables of said
speech decoder if said detected frame is declared non-acceptable in
said deciding step.
15. The method of claim 14, said filter parameter modifying means
including a voice activity detector with at least a threshold for
speech/background sound decisions, wherein said concealing step
comprises restricting updating of said threshold if said detected
frame is declared non-acceptable in said deciding step.
16. The method of claim 15, said parameter modifying means further
including a stationarity detector connected to an output of said
voice activity detector for discriminating between stationary and
non-stationary background sounds, said stationarity detector
including at least one buffer containing estimates of statistical
moments of recent frames dominated by background sounds for making
stationarity decisions, wherein said concealing step comprises
restricting updating of said buffer if said detected frame is
declared non-acceptable in said deciding step.
17. The method of claim 15, said filter parameter modifying means
further including a stationarity detector connected to an output of
said voice activity detector for discriminating between stationary
and non-stationary background sounds, wherein said concealing step
comprises inhibiting updating of the stationari-
ty/non-stationarity decision obtained from the previous frame if
said detected frame is declared non-acceptable in said deciding
step.
18. The method of claim 15, said filter parameter modifying means
including means for low pass filtering of the filter parameters,
said method comprising restricting updating of filter coefficients
of the low pass filtering process if said detected frame is
declared non-acceptable in said deciding step.
19. The method of claim 15, said filter parameter modifying means
including means for bandwidth expansion of the filter represented
by these parameters, said method comprising restricting updating of
filter coefficients if said detected frame is declared
non-acceptable in said deciding step.
20. The method of claim 19, wherein the tilt of the spectrum of the
decoded signal is modified by a post filter, said method comprising
restricting updating of tilt information if said detected frame is
declared non-acceptable in said deciding step.
21. The method of claim 18, wherein the tilt of the spectrum of the
decoded signal is modified by a post filter, said method comprising
restricting updating of tilt information if said detected frame is
declared non-acceptable in said deciding step.
22. The method of claim 14, said filter parameter modifying means
including a voice activity detector for performing
speech/background sound decisions on a frame by frame basis,
wherein said concealing step comprises inhibiting updating of the
speech/background sound decision obtained from the previous frame
if said detected frame is declared non-acceptable in said deciding
step.
23. The method of claim 22, said filter parameter modifying means
further including a stationarity detector connected to an output of
said voice activity detector for discriminating between stationary
and non-stationary background sounds, said stationarity detector
including at least one buffer containing estimates of statistical
moments of recent flames dominated by background sounds for making
stationarity decisions, wherein said concealing step comprises
restricting updating of said buffer if said detected flame is
declared non-acceptable in said deciding step.
24. The method of claim 22, said filter parameter modifying means
further including a stationarity detector connected to an output of
said voice activity detector for discriminating between stationary
and non-stationary background sounds, wherein said concealing step
comprises inhibiting updating of the stationarity/non-stationarity
decision obtained from the previous frame if said detected frame is
declared non-acceptable in said deciding step.
25. The method of claim 22, said filter parameter modifying means
including means for low pass filtering of the filter parameters,
said method comprising restricting updating of filter coefficients
of the low pass filtering process if said detected frame is
declared non-acceptable in said deciding step.
26. The method of claim 11, said filter parameter modifying means
including means for bandwidth expansion of the filter represented
by these parameters, said method comprising restricting updating of
filter coefficients if said detected frame is declared
non-acceptable in said deciding step.
Description
TECHNICAL FIELD
The present invention relates to transmission error concealment in
a radio communication system, and specifically to an apparatus and
a method for improving decoding of background sounds in such a
system.
BACKGROUND OF THE INVENTION
Means for improving encoding/decoding of background sounds,
primarily intended for digital cellular telephone systems, have
been described in Swedish Patent Applications 93 00290-5 and 93
01798-6. These means are primarily designed to handle a situation
where the connection between the speech encoder and the speech
decoder is close to ideal, in the sense that only a small amount of
bit or transmission errors remain after channel decoding. However,
since the connection is a radio channel the received signal may
contain some bit or transmission errors. In such a case it may be
necessary to modify the methods described in the above Swedish
patent applications.
An object of the present invention is an apparatus and a method in
which so called error concealment is applied to the received signal
in order to make the speech decoding more robust or insensitive to
transmission errors.
SUMMARY OF THE INVENTION
In accordance with the invention this object is solved by an
apparatus in a receiver in a frame based radio communication
system, for concealing transmission errors in a speech decoder
caused by a communication channel, which speech decoder is of the
source-filter type and is controlled by means including internal
state variables updated on a frame by frame basis for modifying
received filter parameters representing background sounds
transmitted over said communication channel, said apparatus
comprising:
(a) means for detecting frames containing transmission errors;
(b) means for deciding whether a frame in which transmission errors
have been detected is acceptable;
(c) means for concealing said detected transmission errors by
restricting updating of at least one of said internal state
variables if said detected frame is declared non-acceptable by said
deciding means.
Furthermore, in accordance with the invention the above object is
also solved by a method in a receiver in a frame based radio
communication system, for concealing transmission errors in a
speech decoder caused by a communication channel, which speech
decoder is of the source-filter type and comprises means including
internal state variables updated on a frame by frame basis for
modifying received filter parameters representing background sounds
transmitted over said communication channel, said method
comprising:
(a) detecting frames containing transmission errors;
(b) deciding whether a frame in which transmission errors have been
detected is acceptable;
(c) concealing said detected transmission errors by restricting
updating of at least one of said internal state variables if said
detected frame is declared non-acceptable in said deciding
step.
BRIEF DESCRIPTION OF THE DRAWING
The invention, together with further objects and advantages
thereof, may best be understood by making reference to the
following description taken together with the accompanying drawing,
which is a schematic block diagram of the relevant parts of a
receiver in a radio communication system containing an apparatus in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
To understand the operation of the invention it is useful to
briefly review the operation of a typical digital cellular radio
connection, typical error concealment techniques, and also to
review the algorithms of the above mentioned Swedish patent
applications.
In a communication link of a digital cellular telephone system the
acoustic signal is first digitized and then a speech coding
algorithm is applied (see for example B. S. Atal, V. Cuperman and
A. Gersho, eds, "Advances in Speech Coding", Kluwer Academic
Publishers, 1991). This algorithm compresses the speech signal and
transforms it to a number of quantized parameters (usually in a
frame based manner). The resulting bits are thereafter protected by
addition of coding redundancy, using channel encoding techniques
(see for example G. C. Clark and J. B. Cain, "Error Correction
Coding for Digital Communication", Plenum Press, 1981). The
resulting bit stream is then modulated (see for example J. G.
Proakis, "Digital Communication", 2nd edition McGraw-Hill, 1989)
and transmitted, for example using TDMA (Time Division Multiple
Access) techniques. At the receiver the signal is demodulated.
Possible time or multipath dispersion can be countered by various
equalization techniques, for example Viterbi equalization or
decision feedback equalization (see for example the reference by J.
G. Proakis above). Channel decoding (see for example the reference
by G. C. Clark and J. B. Cain above) is then used in order to
decode the bits that form the quantized parameters that the speech
decoder needs in order to reconstruct the transmitted speech
signal. It is clear from the above discussion that disturbances on
the transmission channel can affect the reconstructed speech
signal, thus reducing the quality of that signal.
Although channel encoding/decoding techniques can reduce the
sensitivity to disturbances significantly, it is usually not enough
to apply only channel coding in a digital cellular system. On the
contrary, it is quite common to additionally use so called error
concealment techniques in order to further mask the perceptual
effects of bit errors remaining at the input of the speech decoder.
These techniques all rely on some information on the quality of the
transmission channel, which information is available or estimated
at the receiving end. When such information indicates that the
quality of the transmission channel is poor, the error concealment
techniques initiate special actions in the speech decoder, with the
purpose of reducing the negative effects of bit errors on the
reconstructed speech signal. The level of sophistication of the
error concealment techniques is dependent on-the character of the
information on the quality of the transmission channel. A few ways
to obtain such information will now be described.
Direct information on the channel quality can be obtained by
measuring the signal strength. A low value would then indicate a
low signal to noise ratio, which means that the channel quality can
be expected to be poor. Channel coding techniques provide a further
level of sophistication. One type of technique is to use redundant
channel coding, e.g. Cyclic Redundancy Check (CRC) (see for example
the reference by G. C. Clark and J. B. Cain above), in particular
when the code is used for error detection. Moreover, "soft" (not
binary quantized) information may be obtained from the
convolutional decoder (in case a convolutional code is used),
demodulator, equalizer, and/or block code decoder (see for example
the reference by J. G. Proakis above). One technique that often is
applied is to divide the information bits from the speech encoder
into different classes, each with different error
correction/detection schemes, thereby reflecting different
importance of different bits (see for example "TR-45 Full Rate
Speech Codec Compatibility Standard PN-2972", Electronic Industries
Association, 1990 (IS-54)). Hence, parts of the information with
error detection/correction codes applied may be used as indicators
of possible bit errors present in the speech frame.
Some techniques to introduce error concealment in conventional
speech decoders with the intention of masking frames that are
considered to contain bit errors will now be briefly described.
When a bad frame is detected, it is common to use the information
from the previous accepted frame. Often this technique is combined
with muting (reduction of output level) in case the bad frame
situation should persist for several frames (see for example "TR-45
Full Rate Speech Codec Compatibility Standard PN-2972", Electronic
Industries Association, 1990 (IS-54)). This situation is not
unusual in mobile telephone systems, where fading dips can persist
for quite long periods of time in cases where the mobile velocity
is low. The result of muting is that disturbances are masked in the
reconstructed signal. In particular loud "clicks" are avoided. When
more detailed information is available on the quality of each
received part of incoming bits, it becomes possible to trace down
possible transmission errors to certain parameters of the speech
decoder. Since said parameters model different phenomena of speech,
error concealment techniques can be developed that are optimized to
the physical meaning of each particular parameter. One particular
example of this is the so called pitch gain (see for example T. B.
Minde et al., "Techniques for low bit rate speech coding using long
analysis frames", ICASSP, Minneapolis, USA, 1993). A value larger
than one is sometimes needed for this parameter during transient
periods of speech. Such a value does, however, correspond to an
unstable filter model, which means that it may be somewhat
dangerous to use it. In particular, it is suitable to introduce
error concealment techniques that limit the pitch gain to values
less than one whenever a possible bit error in that parameter is
detected. A further example is the spectral filter model that is
commonly used in modern speech coding algorithms (see for example
the reference by T. B. Minde et al. above). In that case error
concealment techniques can be used in order to prevent the use of
unstable filters when bit errors are indicated in the corresponding
spectral information. The reverse is also of relevance; whenever an
unstable filter is detected, a bad frame may be indicated and error
concealment techniques may be applied.
With this background information in mind, the present invention
will now be described with-reference to FIG. 1. FIG. 1 shows the
parts of a receiver in a mobile radio communication system
necessary to describe the present invention. An antenna 10 receives
the transmitted signal and forwards it to a demodulator 12.
Demodulator 12 demodulates the received signal and forwards it to
an equalizer 13, for example a Viterbi-equalizer, which converts
the received and demodulated signal into one or several bit
streams, which are forwarded to a channel decoder 14. Demodulator
12 and equalizer 13 also forward "soft" information on received
bits or symbols to a deciding means 16. Channel decoder 14 converts
the bit stream into a filter parameter stream and an excitation
parameter stream for speech decoding. Furthermore, channel decoder
14 performs cyclic redundancy check (CRC) decoding on at least
parts of each received frame. The results of these checks are
forwarded to deciding means 16.
The receiver also contains a speech detector 20 (also called voice
activity detector or VAD). Speech detector 20 determines from said
filter and excitation parameters whether the received frame
contains primarily speech or background sounds. The decision of
speech detector 20 is forwarded to a signal discriminator 22, which
uses some of the excitation parameters to determine whether
received signals representing background sounds are stationary or
not. If a frame is declared as containing stationary background
sounds, the output of signal discriminator 22 controls a parameter
modifier 24 to modify the received filter parameters. This
modification is described in detail in Swedish Patent Application
93 00290-5. The modification may be performed by the parameter
modifier 24 in several ways. One possible modification is a
bandwidth expansion of the filter. This means that the poles of the
filter are moved towards the origin of the complex plane. Another
possible modification is low-pass filtering of the filter
parameters in the temporal domain. That is, rapid variations of the
filter from frame to frame are attenuated by low-pass filtering at
least some of the parameters. Furthermore, the stationarity
detection of signal discriminator 22 and the interaction between
speech detector 20, signal discriminator 22 and parameter modifier
24 is described in detail in Swedish Patent Application 93 01798-6.
The possibly modified filter parameters (if the received signal
represents stationary background sounds) and the excitation
parameters are forwarded to a speech decoder 26, which outputs a
sound signal on output line 28.
In order to describe the error concealment techniques of the
present invention it is necessary to briefly describe the effect of
bit errors on the so called anti-swirling algorithms that are
described in the above two Swedish patent applications. These
effects can be divided roughly as follows:
1. Voice activity or speech detector 20 that is used to control the
anti-swirling algorithm is usually adaptive ("Voice Activity
Detection", Recommendation GSM 06.32, ETSI/GSM, 1991). This means
that there are thresholds and corresponding states that are
automatically updated internally in the voice activity detector,
using either a measured speech signal or, when applied in the
receiver as is assumed here, decoded parameters from the channel
decoder. When there are errors in the incoming parameters this
leads to thresholds or internal state variables that are not
updated correctly, which may result in erroneous decisions. The
result would be a reduced quality of the reconstructed audio
signal.
2. Voice activity or speech detector 20 forms its
speech/back-ground sound decision Using incoming filter and
excitation parameters and also internally updated states, i.e. old
incoming parameters and additional a priori information. Bit errors
may therefore result in immediate erroneous decisions in the
receiver, which leads to reduced quality of the reconstructed audio
signal. Furthermore, since the current decision also depends on old
incoming parameters, bit errors may also influence future
decisions.
3. Signal discriminator 22, which is a part of the system in a
preferred embodiment of the present invention, investigates
incoming statistical moments, preferably energy values that
describe the average signal energy for each frame. It needs to
store a large number of these energies, both for the current frame
and for older frames in one or several buffers (details are
described in the above Swedish Patent Application 93 01798-6).
Should there be errors in these incoming energies, these errors
will also be stored in the buffers causing erroneous decisions for
a significant amount of time. The result would be a reduced quality
of reconstructed background sound signals.
4. The actions that are used in order to counter swirling in case
of a stationary background sound signal suffer in several ways when
an erroneous decision caused by bit errors occurs. One effect is
that speech is destroyed when a stationary background sound is
erroneously detected and the anti-swirling actions are initiated.
The opposite erroneous decision (speech when there is actually a
stationary background sound present) can also occur, and therefore
the character of the background sound is changed momentarily, which
can be rather annoying. Repeated switching between decisions
indicating speech/-stationary background sounds is also
undesirable, since time constants are in effect during the
transition between the two states. Should excessive switching take
place because of bit errors, this would be very disturbing.
5. The actual anti-swirling actions in parameter modifier 24
(essentially spectral low pass filtering in combination with
bandwidth expansion, as explained in detail in the above Swedish
Patent Application 93 00290-5) suffer from bit errors. One effect
occurs because of erroneous decisions from speech or voice activity
detector 20 or from signal discriminator 22. In these cases the
updating of the low pass filter may be initiated or turned off,
causing a deviation as compared to the case of a perfect channel.
Another effect occurs when there are bit errors affecting the
spectral information that feeds the low pass filters and the
bandwidth expansion. Both these effects may cause quality
reductions.
6. A postfilter of speech decoder 26 (whenever present) has similar
problems as described in section 5. above. Furthermore, the so
called spectral tilt is perceptually very sensitive, and since it
may be manipulated by the anti-swirling algorithm, bit errors can
give a significant quality reduction of the reconstructed speech
signal.
7. The effects described above may combine and influence each
other. For example an erroneous decision in speech detector 20 may
result in stopped updating of buffers of signal discriminator 22.
This in turn will influence signal discriminator 22 for a
significant period of time, causing reduced quality of the
anti-swirling actions.
From the above discussion it is clear that transmission errors can
result in incorrect updating of internal variables of speech
detector 20, signal discriminator 22, parameter modifier 24 or
combinations thereof. In accordance with the present invention
these problems are reduced or eliminated by modifying the updating
process during frames in which transmission errors have been
detected. This modified updating will now be described more in
detail.
Deciding means 16 receives "soft" information on the reliability of
bits from demodulator 12 and equalizer 13, and the results of
CRC-checks from channel decoder 14. Deciding means 16 then
determines whether bit errors have occurred or are likely (from the
"soft information"). Furthermore, it is decided whether possible
bit errors influence the filter or excitation parameters. If this
is the case, and if these bit errors occur in parameters that
significantly influence the operation of speech detector 20, signal
discriminator 22 or parameter modifier 24, a corresponding signal
is forwarded to concealing means 18. This can for instance be the
case if the parameter corresponding to the frame energy contains
one or more bit errors.
Depending on the parameter that contains bit errors, concealing
means 18 can control the operation of the updating process of
corresponding internal state variables in speech detector 20,
signal discriminator 22 and parameter modifier 24, as indicated by
control lines 30, 32 and 34, respectively. These modifications
include:
Stopping updating of internal state variables (for example
thresholds) of speech detector 20 when a bad frame is detected.
This implies that the internal variables of speech detector 20 are
locked to the same value as in the previous frame, or that the
updating of these state variables is restricted (state variables
can only be updated by smaller amounts than usual).
Another action is to lock the decision in speech detector 20 to the
decision from the previous frame in case a non-acceptable frame
containing transmission errors is detected.
If a non-acceptable frame containing transmission errors in
excitation parameters that are relevant for stationarity decisions
has been detected, the updating of the buffers in signal
discriminator 22 can be stopped or restricted.
Another possibility is to lock the decision of signal discriminator
22 to the decision of the previous frame.
If the received frame contains transmission errors in bits
containing spectral information the updating of internal filter
coefficients of parameter modifier 24 controlling low pass
filtering and/or bandwidth expansion can be stopped or
restricted.
The spectral tilt of a possible post filter can be locked to the
tilt of the previous frame.
Since different received parameters influence different blocks in
FIG. 1 (speech detector 20, signal discriminator 22, parameter
modifier 24 and a possible postfilter), it is appreciated that one
or several of these actions can be taken, depending on where in the
received frame the bit errors have been detected.
In the above discussion it is also appreciated that if transmission
errors occur in a certain received parameter for consecutive
frames, the corresponding internal state variable of the
corresponding block of the receiver will be locked to (or
essentially locked to) its value in the latest received frame in
which the corresponding parameter was correctly received.
A preferred embodiment of the method in accordance with the present
invention is illustrated in detail by the two PASCAL program
modules in the attached APPENDIX.
It will be understood by those skilled in the art that various
modifications and changes may be made to the present invention
without departure from the spirit and scope thereof, which is
defined by the appended claims. ##SPC1##
* * * * *